Probe for measuring workpieces

ABSTRACT

A contact-sensing probe for measuring workpieces includes a body (16) for attachment to a machine and a workpiece contacting stylus (22) carried by a stylus holder (24) which is mounted within the body for movement relative thereto from a rest position to a displaced position when the stylus contacts the workpiece. Sensing means are provided for sensing workpiece contact by the stylus. The sensing means comprise a plurality of strain gauges (46) mounted on pillars (44) connecting two parts (36A,36B) of a support structure which supports the stylus holder on the body. The strain gauges are mounted with their principal axes lying at an angle to the axes of the pillars, and by optimizing the numbers, positions and dimensions of the pillars together with the angles of the strain gauges, high sensitivity can be achieved with minimum risk of false signals due to probe vibration, while at the same time minimizing the variation in pre-travel of the stylus.

This invention relates to a probe for measuring the dimensions ofworkpieces.

It is known to measure the dimensions of workpieces in co-ordinatemeasuring machines or machine tools by providing measuring apparatusincluding a measuring probe for determining the co-ordinate position ofthe probe relative to the workpiece. The apparatus is operated to movethe probe towards a surface of the workpiece whose co-ordinate positionis to be measured and the probe is adapted to output a sensing signalresponsive to a stylus, forming part of the probe, engaging the surface.In so-called "trigger probes" the probe sensing signal is a step signalproduced responsive to engagement between the stylus and the surface,and the position of the surface is measured in terms of the reading ofthe measuring devices of the machine at an instant following saidengagement. The step signal is produced as a result of the stylus, whichforms part of an electric circuit of the probe, being displaced from arest position which causes a change of state of the circuit.

In known trigger probes there is a difficulty in accurately relating theinstant at which the stylus engages the workpiece to the instant atwhich the step signal is received by the machine. The unavoidabledisplacement of the stylus between these two events (the "pre-travel" ofthe stylus) is not always uniform for all conditions of operation. Inparticular, the pre-travel may be different for different directions ofdisplacement of the stylus relative to the base. Since the measurementby the measuring devices of the machine is carried out while the probeis in motion, then, for a given speed of that motion, any variation inpre-travel results in a variation in the measurement.

In our European Patent Specification No. 0068899 an attempt was made todeal separately with the displacement of the stylus which is requiredfor producing the probe sensing signal, and the displacement (or"overtravel" of the stylus) required to prevent damage to the probe orthe machine before the machine can be stopped after the sensing signalhas been generated. In this Patent specification there is disclosed asensing system comprising strain-sensitive elements disposed at threelocations around the axis of the stylus, between an intermediate memberand the body of the probe. An strain-sensitive elements are used toprovide an early indication of movement of the intermediate memberrelative to the base before the stylus which is also mounted on theintermediate member, is displaced from its rest position.

Although this arrangement reduces the amount of pre-travel of the stylusthere are still variations in pre-travel depending on the differentdirections of displacement of the stylus by the workpiece, particularlywhen using styli which make contact with the workpiece at a point offsetfrom the axis of the probe.

Another attempt to provide greater accuracy of measurement using a probeis disclosed in our published International Patent Specification No. WO85/04706. In this specification there is described the concept of usingdisplacement sensors connected to the stylus and generating a singlesensing signal for communicating with a machine only when the combinedsignal level from the sensors has reached a pre-determined thresholdlevel indicating a pre-determined displacement of the stylus. Then bysuitable programming of the machine computer, the known displacement canbe taken into account and the machine is arranged to indicate theposition of the probe prior to such displacement i.e. the position offirst contact with the workpiece.

There are however, some limitations on this system in that it is onlyviable with a stylus which is in line with the axis of the probe. Alsowith the type of transducer shown, the probe is relatively expensive,requires a new calibration for a stylus of different length, and hasonly limited overtravel protection.

It is an object of the present invention to provide a probe formeasuring workpieces which reduces variations in stylus pre-travel whichhave occured with previous probe systems.

It is another object of the invention to provide a probe which allowsthe actual pre-travel to remain at an acceptably high value to providehigh immunity to false triggering from external vibration oracceleration of the machine. It is a further object of the invention toprovide a probe which operates in six mutually perpendicular directionsand achieves the first two objects specified above in all sixdirections.

The invention as claimed reduces the variation in pre-travel in a probeby providing support structure for supporting the probe stylus on theprobe body and which comprises two portions interconnected at positionsequally spaced around the probe axis by pillars each of which has alongitudinal axis which is parallel to the axis of the probe. Oneportion of the structure is connected to the probe body and the stylusis supported on the other portion. Elongate strain sensitive elementsare mounted on each of the pillars with their axes inclined to thelongitudinal axes of the pillars.

The advantage of this invention is that the dimensions and positions ofthe pillars, and the positions and orientations of the strain sensitiveelements in relation to the pillars, can be optimized to produce notonly the required pre-travel of the stylus for immunity to falsetriggering, but one which has a very small variation regardless of thedirection of displacement of the stylus, even with styli having axesinclined to, or offset from, the axis of the probe.

One example of the invention will now be described in detail withreference to the accompanying drawings in which:

FIG. 1 is a sectional elevation through a probe according to the presentinvention.

FIG. 2 is a plan view on the line II--II of FIG. 1 showing the manner inwhich the moveable parts of the probe are supported.

FIG. 3 is an enlarged detailed illustration of one of thestrain-sensitive elements of the present invention on its associatedpillar.

FIG. 4 is a circuit diagram of the electronic elements of the probesignalling system.

FIG. 5 is a circuit diagram showing the connections between the probeand an interface unit on a machine.

Referring now to FIGS. 1 to 3 there is shown a measuring probe 10 whichis intended to be supported by movable slides of a measuring machine ormachine tool (not shown in detail) for movement relative to a workpiece12 positioned on the base 14 of the machine. The machine slide movementbrings the probe into engagement with a surface of the workpiece 12 forsensing the position thereof, and on engagement with said surface theprobe sends a signal to the machine which then determines theco-ordinates of the position of the probe in space by recording thepositions of the slides of the machine using position counters providedon the machine.

The probe has a body 16 which has a shank 18 for connection to themachine. The body can thus be regarded as comprising fixed structure ofthe probe. The body has an axis 16A.

Supported within the body 16 is a movable structure, indicated generallyby reference numeral 19, which includes a stylus holder 20 to which astylus cluster 21 is connected. The stylus cluster in this example hasfive styli 22 extending in mutually perpendicular directions from thecenter of the cluster. The five styli enable contact to be made betweenthe probe and various surfaces of a workpiece by machine movement in anyof six mutually perpendicular directions.

The stylus holder 20 incorporates a triangular center plate 24 (FIG. 2)centered on the axis 16A and having three seat elements 26 disposed oneat each of its apices in the form of rollers which extend radially ofthe axis 16A. The plate 24 is supported in a rest position on anintermediate member 28 by means of pairs of seat elements 30 carried bythe intermediate member 28 which engage the rollers 26 and formtherewith a kinematic support for the plate 24 on the intermediatemember 28. A spring 32 in tension urges the plate 24 into the restposition on the seat elements 30 which ensures a positive location ofthe stylus holder. The stylus holder can however be tiltably or axiallydownwardly displaced against the spring force from the rest positionwhen any one of the styli 22 engages the workpiece 12.

The intermediate member 28, which also forms a part of the movablestructure 19, comprises a triangular plate which is supported on theprobe body 16 by a kinematic support which comprises the spherical seatelements 30 on the intermediate member and three pairs of seat elements34 in the form of radially extending rollers 34 on an annular rollerplate 36A. The seat elements 30 are urged downwardly into engagementwith the rollers 34 by a spring 38 to define a positive location for theintermediate member on the plate 36A from which it can be tiltably oraxially displaced against the spring force when any one of the styli 22engages the workpiece. In this example the spring 38 is located by aconical spring support 42 which in turn reacts on a triangular fixedstructure 40, connected to the plate 36A, the arrangement being suchthat the spring 38 is normally in compression. Plate 36A is the solemeans of support for the intermediate member 28.

The plate 36A forms one portion of an annular fixed structure 36 ofwhich a second portion 36B is connected at its radially outer peripheryto the probe body 16.

The two portions 36A and 36B of the structure 36 are interconnected byat least three circumferentially spaced pillars 44. The pillars 44 forma region of relative weakness in the load path between the stylus holderand the probe body so that they form the areas of greatest strain inthat load path when a force is applied to the stylus. The pillars havean axis 44A which is parallel to the axis 16A of the probe body.

Sensing devices 46, in the form of elongate semi-conductor strain gaugesare provided, one on each pillar, and each strain gauge is positionedwith its longitudinal axis 46A inclined at an angle θ to the axis 44A ofthe pillar. By this means any one of the strain gauges will provide asignal when any strain is put into the pillars whether the strain istensile, compressive or torsional, or any combination thereof.

The dimensions d,b,and 1 of the pillars and their numbers and positionsaround the structure 36, together with the inclinations of the straingauge axes are optimized to provide, the highest sensitivity compatiblewith the conflicting needs of avoiding false triggering due to machinevibrations and accelerations, and of providing the smallest variation inpre-travel under all directions of application of force to any one ofthe styli 22.

In one practical probe construction three pillars each 0.5mm long wereequally spaced around the structure 36 and to each pillar was attached asemi-conductor strain gauge, the longitudinal axis of which was inclinedat 25 degress to the axis of its respective pillar. With this probe itwas possible to achieve a trigger signal from the stylus after a movmentof two microns, plus or minus 0.5 microns, in any direction ofapplication of force to the end of any one of the styli 22, regardlessof whether the stylus involved was positioned horizontally orvertically, or in any other position within a hemispherical envelope,indicated at 51, centered at 0. The lower part of the probe body 16 wassurrounded by a sleeve 48 and sealed with a rubber seal 49 forprotection against damage and ingree of dirt.

By using sensitive strain gauges and short pillars, the relativestiffness of the structure 36 is high which produces the high immunityto false triggering and provides for negligible mechanical hysteresis inthe system.

In operation, the probe is driven by the machine towards a surface ofthe workpiece 12. Using a cluster stylus having five orthogonal styli22, measurements can be made in any one of six orthogonal directions.Because the supports for the stylus holder on the intermediate memberand for the intermediate member on the structure 36 are kinematicsupports, the whole of the movable structure and the structure 40 can beregarded as a single solid structure until such time as the forces ofthe springs are overcome by the displacing force on the stylus when itmakes contact with the surface of the workpiece.

Thus on an initial contact with the workpiece, any deflection of thestylus results in a strain being put on the single solid structure,which is greatest at the pillars, and is sensed by the strain gauges 46.Signals from the strain gauges are passed by wires 47 to an electricalcircuit 50 within the probe body, in which the changing resistance ofthe strain gauges is sensed. The electrical circuit 50 produces atrigger signal which is passed to a second electrical circuit of theapparatus external to the probe (which in this example is in theinterface unit IF FIG. 5) where the signal is processed before beingpassed to the measuring devices of the machine, to read theinstantaneous position of the probe, and to stop the machine. Since themachine cannot be stopped instantaneously its continued movement causesfurther deflection of the stylus until the force of one or other of thesprings is overcome, and the stylus holder, or the intermediate memberlifts of its respective kinematic support. This provides adequatebraking movement of the machine in all six directions before any damageto the probe results.

As an optional fail-safe arrangement, displacement of the stylus fromits rest position may be additionally sensed by an electric circuit 49which serially connects all of the spherical elements 30 thus providingswitch contacts which are completed by the seat elements 26,34 in therest position. Thus, any displacement of the stylus causing lifting ofany of the seat elements from its respective contacts breaks thecircuit. The circuit is connected to the electronic circuit 50 whichsenses this change of state.

Although the above-described probe has a stylus capable of movement insix directions along the orthoganal axes ±x,±y and ±z, it will beunderstood that the invention could also be applied to a probe having astylus capable of movements only in five directions, i.e. one in whichthe stylus cannot move in the vertically downward direction (the -zdirection).

Also it will be understood that the locations and relative dispositionsof the seat elements on the static and movable structures may be variedto provide alternative arrangements of the kinematic supports withoutdeparting from the basic principle of the invention.

Details of the electronic circuit 50 will now be described withreference to FIG. 4. The three strain gauges SG1, SG2 and SG3 areconnected with resistors R1, R2 and R3 between a supply voltage Vs whichoriginates from a DC source VDC and a reference voltage Vo connectedrespectively to the interface unit IF at terminals T1 and T5. The supplyvoltage is regulated to a constant value by voltage regulator VR. Thevalue of each resistor is arranged to be the same as the nominalresistance of each strain gauge so that the nominal voltages atjunctions J1, J2 and J3 between the resistors and the strain gauges is0.5Vs. Any change in the resistance of the strain gauges causedvariations of the voltage at the three junctions and these variationsare amplified in amplifiers A1, A2 and A3 which provide outputs AO1, AO2and AO3 which are passed to window comparators W1 W2 and W3 whichprovide the trigger signal from the probe to the machine. In order toavoid false triggering signals due to variations in the resistance ofthe strain gauge from their nominal value, due to tolerances inmanufacturing specifications or drift due to atmospheric conditions, anauto-zeroing circuit is provided which biasses the output of each of theamplifiers to 0.5Vs. The operation of the auto- zeroing circuit isdescribed with reference to one only of the amplifiers e.g. A1. For thepurposes of this description 0.5Vs is taken to be zero.

The voltage output A01 of the amplifier A1 is connected as one of thevoltage inputs of a transconductance amplifier TC1, the other voltageinput for which is 0.5Vs.

A current input Ia is also provided for amplifier TC1, and the outputTC01 of amplifier TC1, which is a current output, is dependent on thedifference between the voltage inputs A01 and 0.5Vs, and on the currentinput Ia. The operation of the transconductance amplifier TC1 is suchthat when either this voltage difference or the current IA is zero, theoutput TC01 is zero. Output TC01 is connected both to a potential of0.5Vs through a capacitor C1, and to one of the inputs of amplifier A1.

A high impedance is provided at the input of amplifier A1 so that thecurrent output TC01 flows into the capacitor C1 changing its potentialand providing a voltage input at amplifier A1. It will be seen thatwhile the output from amplifier A1 remains at zero the output TC01 fromthe amplifier TC1 will be zero, but any change in the resistance of thestrain gauge will cause a change in voltage at junction J1 which willproduce an output A01 from amplifier A1. This in turn causes an outputfrom amplifier TC1 which produces a correcting voltage input toamplifier A1 from capacitor C1 which is arranged to be such as to biasthe output of amplifier A1 to zero. Similar components being referencedby the same letters with suffices 2 and 3 make up auto-zeroing circuitsfor strain gauges SG2 and SG3 respectively and these circuits are notdescribed in detail. Once the outputs from amplifiers A1,A2 and A3 havebeen stabilized, any change, in any direction, of any one of the straingauge outputs causes a change in the respective amplifier output. Thischange, if greater than the threshold values of the window capacitorsW1,W2 and W3 which are nominally set at +100 mV and -100 mV, will causethe window comparators to switch from logic high to logic low regardlessof whether the change in amplifier output was positive or negative. Anyone of the comparators going "low" will make the junction JC where theoutputs of all of the comparators are interconnected go "low". The "low"signal is passed through an inverter I to provide a conventional logic"high" output from the electronic circuit as the "probe triggered"signal which is coupled to terminal T2.

When the probe is switched on its is desirable that the auto-zerocircuits work relatively rapidly to stabilize the outputs of amplifiersA1, A2 and A3, but it is important that during probing, the signals fromthe amplifiers are not nullified by the auto-zero circuit before theyreach the threshold level required to trigger the window comparators.The speed of operation of the auto-zero circuit must therefore bevariable and this is achieved by varying the current Ia from a highlevel to a low level.

For the high speed mode of operation a start-up circuit S provides asteady high level d.c. current for about 3 seconds causing a highcurrent inputs 1a to each of the transconductance amplifiers. Thus whilethere is an output from any of the amplifiers A1, A2 or A3, the outputsof the respective amplifiers TC1, TC2 or TC3 will be high causing arapid charging of capacitors C1 C2 or C3 to provide voltage inputs tothe amplifiers A1, A2 or A3 to nullify any inputs from the junction J1,J2 and J3. After the three second interval the start up circuit is cutout automatically.

For low speed operation the current Ia is provided from an oscillatorcircuit OC. This circuit may be of any known type which is capable ofproviding d.c. current pulses at a high mark space ratio, such as toreduce the average current level Ia by about 550:1. The oscillatoroutput is fed to a summing junction J4. Thus the auto-zeroing currentinput is only available in short pulses between which, the outputs fromthe transconductance amplifiers are zero. The capacitors thus receivecurrent is short-bursts when there is an output from amplifiers A1, A2and A3 and the rate of charging of the capacitors is greatly reduced.The reduction is aimed to give a low rate of auto-zeroing which is aboutone tenth of the speed at which the strain gauge outputs change duringprobing.

The advantage of this method of reducing the current Ia is that itavoids the need for high value resistors which would be needed toprovide a low current Ia from the d.c. voltage Vs. The low averagecurrent produced by the oscillator allows the use of small low valuecapacitors while providing a low average current.

After the probe has been triggered the autozero circuits are inhibitedto prevent them from reducing the outputs of amplifiers A1, A2 and A3 toa value below the threshold values of the window comparators while thestylus is deflected. This would give a false indication to the machinethat the movable structure had returned to its rest position, and wouldprovide a false trigger signal when the stylus is dis-enaged from theworkpiece and the movable member returns to its rest position. Toinhibit the autozero circuit a connection IN is made from junction JC toprovide a signal to the oscillator circuit OC to inhibit its operationwhen a trigger signal is produced by the window comparators. When themovable structure of the probe returns to its rest position and thewindow comparator outputs change state from low back to high, the changein output signal automatically releases the inhibit signal.

An external re-set capability is required once the inhibit signal hasbeen initiated in case the probe is left for too long with its stylus inthe deflected position. In this situation the charge may leak away fromthe capacitors C1, C2 or C3 changing the voltage input to the respectiveamplifiers A1, A2 and A3 so that when the probe is pulled away from theworkpiece and the stylus returns to its rest position the circuit maycontinue to indicate a triggered condition. Thus, a re-set circuit RS isprovided so that whenever the probe has been deflected for more than 10seconds the re-set circuit may be activated by a signal RS1 tore-introduce the high level d.c. current Ia to activate the fastautozeroing condition.

One other addition to the circuit 50 is a threshold control circuit TCfor the window comparators, which enables them to be switched from highgain to low gain when the probe is being moved rapidly betweenoperations to avoid surges through the circuit due to shock loads on theprobe. This circuit along with circuit s is activated by a signal TCSfrom the interface unit.

The optional circuit 49 may be included to provide the additional stylusdisplacement signal at terminal T2.

The circuit 49 is connected at one end to reference voltage Vo and atthe other end to the inverter I via resistor RD. A pull-up resistor RPis connected between resistor RD and the supply voltage Vs. The value ofresistor RD is chosen to provide a new reference voltage to the inverterof 1/2V when the circuit 49 is included. When the stylus is displacedand the circuit 49 goes open circuit, the pull-up resistor RP pull theinverter reference voltage up to Vs to provide the signal for themachine.

In one embodiment of the invention operation of the re-set circuit RSand the threshold control circuit TC is achieved from control circuitsof the machine or interface unit (not shown but know per se). Thesecircuits operate conventionally at a voltage less than the supplyvoltage Vs and require separate external connections to be made to theprobe at terminals T3 and T4. All communications between the probe andthe machine or interface to take place using only two externalconnections to the probe. In this way the probe can be made to beinterchangeable with conventional probes.

In order to achieve this the connections between the probe and theinterface unit are modified as shown in FIG. 5 in which it can be seenthat terminal T1 is connected directly to a terminal ET1 and terminal T5is connected to a terminal ET2. ET1 and ET2 form only two externalterminals on the probe to which a supply and return connections are madeto the interface. The probe-triggered signal from terminal T2 isconnected through an electronic switch unit ES and load resistor RL toterminal ET1. Terminal T3 is connected to the reference voltage line Voat terminal ET2, thus effectively incapacitating the re-set circuit, andthe terminal T4 is connected to the supply upstream of terminal T1, i.e.upstream of the voltage regulator VR.

By this means all of the two-way communication betweeen the probe andthe interface unit is achieved as follows:

With the supply voltage at its nominal level Vs the probe is arranged tobe in high gain mode and a `quiescent` current Iq flows around the probeand to the inferface unit. When the probe is triggered the signalactivates electronic switch ES which causes a further current to flowthrough the load resistor RL. This increased current is detected in theinterface unit as an increase in voltage across a low value resistor Rin series with the supply. A comparator C in parallel with resistor Rdetects the increase and produces and output CO which is suitablyconditioned in the interface for receipt by the machine to stop themachine and take the measurement readings. When the probe stylus returnsto the rest position the switch ES is de-activated and the current flowreturns to Iq.

When a change in gain is required at the window comparators the voltagefrom the supply is increased by a nominal 3 volts to Vs1 to providesignal TCS at terminal T4 to activate the threshold control TC. Becauseterminal T4 is connected to the supply upstream of the voltage regulatorVR, the circuit downstream of VR is not affected since regulator VRmaintains a constant voltage VS into the probe circuit and the quiescentcurrent Iq is not affected.

To obtain a re-set action the supply voltage is reduced to near zero fora short period and then returned to the level of Vs or the higher Vs 1.This activates the start-up circuit S which provides the re-set action.

One electrical circuit for achieving the changes in voltages is shown atthe right hand side of FIG. 5 in which a further voltage regulator VR2having first input

I/P and an output of O/P is connected between resistor R and the sourceVDC. The regulator is of conventional construction e.g. as sold underthe designation LM317 and is pre-programmed to provide the three voltagelevels required depending on the change of state of switches S1 and S2which vary the voltage applied to the sensing input SI of the regulator.The switches are opened or closed under the control of the machinecontrol computer MC which determines the need for a voltage change underany given circumstances of operation of the machine. The componentsreferenced R10, R11 and R12 are resistors which complete the circuit.

We claim:
 1. A probe for measuring workpieces comprising:a body having an axis, a fixed structure contained within the body, said fixed structure having a first portion rigidly connected to the body, and a second portion spaced from the first portion in the direction of said axis of said body, said two portions being interconnected by a relatively weak structure which becomes strained when a load is applied to the fixed structure, a movable structure at least partially contained within the body and including a stylus holder to which one or more workpiece-contacting styli are connectable, support means for supporting the movable structure in a rest position on the second portion of the fixed structure, bias means for urging the movable structure into the rest position from which it is displaceable in opposition to the bias means when a stylus contacts a workpiece and a displacing force is thereby produced on the stylus, the bias means acting to restore the movable structure to the rest position when the displacing force ceases, sensing means for sensing said stylus contact with the workpiece and for providing an electrical signal indicative thereof, and an electrical circuit for processing said signal, wherein the sensing means comprises at least three elongate strain sensitive elements having longitudinal axes and which are mounted on the relatively weak structure at equally spaced locations around said axis of the body with their longitudinal axes lying obliquely to said axis of the body.
 2. A probe as claimed in claim 1 wherein the relatively weak structure comprises at least three pillars extending between the first and second portions of the fixed structure at locations equally spaced about said axis of the body, and at least one of the strain sensitive elements is mounted on each pillar.
 3. A probe as claimed in claim 1 wherein the fixed structure further comprises a part connected to one of the first or second portions against which the bias means reacts whereby the forces produced by the bias means are not reacted through the relatively weak structure.
 4. A contact sensing probe as claimed in claim 1 wherein the support means comprises a pair of support elements on one of the movable structure or the fixed structure at each of three locations equally spaced apart around said axis of said body and confronting support elements on the other one of said structures at each of the three locations.
 5. A contact sensing probe as claimed in claim 4 wherein the movable structure comprises a stylus holder and an intermediate member, the intermediate member is urged into said rest position on the fixed structure at said support means by said bias means, further support means are provided at three locations equally spaced apart around said axis of said body for supporting the stylus holder in a rest position on the intermediate member and further bias means are provided for urging the stylus holder into to rest position at said further support means.
 6. A contact sensing probe as claimed in claim 1 wherein each of the strain sensitive elements are connected with an additional resistor in an electrical path between a DC voltage supply and a refernce voltage to form a junction which is connected to a window comparator which produces a pulse signal when a variation in voltage at the junction exceeds a predetermined level, which pulse signal constitutes the output of the probe.
 7. A contact sensing probe as claimed in claim 6 wherein an auto-zeroing circuit is connected between each junction and the associated window comparator to bias the variation in voltage towards zero to compensate for drifting of the resistance of the strain gauge.
 8. A contact sensing probe as claimed in claim 5 wherein the support means and the further support means are provided at the same three locations. 